Uqw I Salt Effect on Transient Proton Transfer to Solvent and Microscopic Proton Mobility

Salt Effect on Trans i en t P r o t o n Transfer to So lvent and M i c r o s c o p i c P r o t o n M o b i l i t y Noam Agmon Department of Physical Chemistry and the Fritz Haber Research Center, The Hebrew University, Jerusalem 91904, Israel Sinay Y. Goldberg and Dan Huppert School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Ramat-Aviv, Tel-Aviv 69978, Israel Received 21 March 1995 ; accepted 5 July 1995 ABSTRACT The effect of an inert salt on the time-resolved fluorescence from excited hydroxypyrene-trisulfonate is measured and compared quantitatively to the solution of the transient Smoluchowski equation for proton diffusionreversible recombination in the excited-state. The data in water can be explained by the screening and mobility-diminishing effects of the added salt. In methanol-water mixtures we observe additionally an enhancement of proton dissociation. The microscopic mechanism for proton mobility in water and methanolic mixtures is discussed. Since the rate-limiting step for proton mobility is a hydrogen-bond cleavage, percolation properties of the hydrogen-bond network might lead to a discontinuous drop in proton mobility, possibly reflected in our data.